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Tech Tip: Dereference Variable Names Inside Bash Functions

Jan 15, 2010  By Mahaveer Darade
 in

We often read (including in the book Advanced Bash-Scripting Guide by Mendel Cooper) that if we pass variable names as parameters to functions, they will be treated as string literals and cannot be dereferenced (ie the value is not available). But this is not so, variable names can be passed as parameters to functions and they can be dereferenced to obtain the value of the variable with the given name.

The following script demonstrates this: 

DerefernceVariablePassedToFunction() {
    if [ -n "$1" ] ; then
        echo "value of [${1}] is: [${!1}]"
    else
        echo "Null parameter passed to this function"
    fi
}


Variable="LinuxJournal"
DerefernceVariablePassedToFunction Variable

If we put above code in a file and run it, we will get output as: 

$ /bin/bash DereferenceVarInFunction.sh
value of [Variable] is: [LinuxJournal]

The secret here is the "!" used in the variable expansion "${!1}". The bash manual states:

If the first character of parameter is an exclamation point, a level of variable indirection is introduced. Bash uses the value of the variable formed from the rest of parameter as the name of the variable; this variable is then expanded and that value is used in the rest of the substitution, rather than the value of parameter itself. This is known as indirect expansion.
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re: what are you using it for?

geofs's picture
Submitted by geofs (not verified) on Mon, 01/18/2010 - 05:26. 
# i've got a list of directories, like this:

dir_app1="/opt/config/app1"
dir_app2="/opt/config/app2"
dir_app3="/opt/config/test/app_3"

# i then create the directory structure
# the trick here is ${!dir_*} that expands to the name
# of all variables beginning with dir_

for i in ${!dir_*}; do
    echo "  making directory ${!i}"
    mkdir -p ${!i}
done

# then i change access rights
chmod 755 $dir_app1
chmod 700 $dir_app2
...

It has been described already for ages, especially in ABS guide.

przemoc's picture
Submitted by przemoc (not verified) on Sat, 01/16/2010 - 05:05.

It's a falsehood stating ABS guide teaches that variable names passed as parameters to functions cannot be dereferenced. Below you can find more and less portable examples in ABS guide.

http://tldp.org/LDP/abs/html/abs-guide.html#INDREF
http://tldp.org/LDP/abs/html/abs-guide.html#INDFUNC
http://tldp.org/LDP/abs/html/abs-guide.html#DEREFERENCECL
http://tldp.org/LDP/abs/html/abs-guide.html#REFPARAMS
http://tldp.org/LDP/abs/html/abs-guide.html#EMBARR

Good practice needs verifying whatever you want to write and publicize. Remember about that next time!

Nice but won't work if text is the same as a variable name

PerfMonk's picture
Submitted by PerfMonk on Fri, 01/15/2010 - 23:02.

In your example, if you call :
DerefernceVariablePassedToFunction "Variable"

It will return "value of [Variable] is: [LinuxJournal]" ...

Unexpected since we gave it the text "Variable" not the variable...

Sorry I have not enough knowledge in bash scripting to suggest a better solution.

Regards,

BT

Strings are strings

fest3er8's picture
Submitted by fest3er8 on Sat, 01/16/2010 - 01:42.

Ah, but "Variable" is the same as Variable, as far as the shell is concerned; all parameters are strings. You don't pass a variable name (symbol) to a function; you pass a string.

So

DerefernceVariablePassedToFunction "Variable"

is the same as

DerefernceVariablePassedToFunction Variable

or...

lingrlongr's picture
Submitted by lingrlongr (not verified) on Fri, 01/15/2010 - 15:38.

something like this could be used for other shells, like KSH, and may be more portable...

#!/usr/bin/ksh

deref()
{
    if [ -n "$1" ]; then
        localvar=$(eval echo \$$1)
        echo "value of [${1}] is: [${localvar}]"
    else
        echo "Null parameter passed to this function"
    fi
}


var="some text to display"

deref var
deref

this will display:

value of [var] is: [some text to display]
Null parameter passed to this function

great info

k.sangeeth's picture
Submitted by k.sangeeth on Sat, 01/16/2010 - 14:32.

..I already started using it :)

Sangeeth Keeriyadath
www.sangeek.com

thx

lingrlongr's picture
Submitted by lingrlongr (not verified) on Sun, 01/17/2010 - 13:42.

i've used this on numerous occasions where my code will dynamically construct a "structured" variable name then retrieve the value for that variable, if it is set.

for instance, if i have a list of database tables that need to be reorganized and i want to reorganize against a particular index, i'll set up a config file:

#!/bin/bash

# these values may be in another file and sourced in
reorg_my_table_1_index="id"
reorg_my_table_2_index="location"

deref()
{
    if [ -n "$1" ]; then
        localvar=$(eval echo \$$1)
        echo "value of [${1}] is: [${localvar}]"
    else
        echo "Null parameter passed to this function"
    fi
}

for tab in my_table_1 my_table_2 my_table_3; do
    echo $(deref "reorg_${tab}_index")
done

what are you using it for?

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